Democratic presidential candidates Barack Obama and Hillary Clinton lead all the top Republican candidates in separate head-to-head match-ups for the 2008 election, according to a CNN/Opinion Research Corp. poll.

Illinois Senator Obama received 49 percent support over Republican Arizona Senator John McCain's 48 percent; 56 percent support compared with 40 percent for former New York Mayor Rudy Giuliani; 59 percent against former Massachusetts Governor Mitt Romney's 37 percent; and 58 percent compared with 39 percent for former Arkansas Governor Mike Huckabee.

New York Senator Clinton received 50 percent over McCain's 48 percent in a head-to-head match-up; 58 percent support against Romney's 40 percent; 55 percent compared with Giuliani's 42 percent; and 56 percent compared with 42 percent for Huckabee.

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